1. Field of the Invention
The invention relates to the field of solar concentrator photovoltaic monolithic microarrays, and more particularly to the lens used in such arrays.
2. Description of the Prior Art
Commercially available solar cells have efficiencies ranging from 8% to 15%. The current world record is 29% efficiency in normal operating conditions and 32.6% efficiency under concentration. It is well known that concentrating solar energy onto the photovoltaic cell increases the cell""s efficiency. However, such lenses when made of glass and the arrays made from them are costly in large arrays.
The fabrication of solid immersion lenses from polydimethylsiloxanes (General Electric RTV615) is well known and shown by U.S. Pat. No. 6,301,055 (2001) for use in imaging systems, and the combination of a focusing lens with a glass solid immersion lens for use in a recording apparatus, typically compact disc players or writers is described for example in U.S. Pat. No. 6,369,957 (2002).
What is needed is some kind of a method for producing more efficient solar cells at lower cost per watt of solar energy.
The invention is directed to a non-imaging mirrorless optical lens system that serves as concentrator lenses for concentrated-sunlight photovoltaics applications using polydimethylsiloxanes (PMDS or GB Silicones RTV615A and B) using soft lithography techniques. The microconcentrator lens arrays provide the performance advantages of concentrator photovoltaics, e.g. higher cell efficiency, smaller cell absorber area, which are realized while preserving the overall sheet-like solar module geometry that is typically employed in nonconcentrator photovoltaic systems. Notably, the reduction in total cell absorber area enables economically competitive terrestrial photovoltaic module designs that utilize cell components with relatively high cost/area, such as compound semiconductor tandem multifunction solar cells and high efficiency silicon solar cells.
The fabrication of microconcentrator lenses according to the invention first requires the preparation of molds and subsequently solid immersion lenses and Fresnel lenses. The molds for these lenses are composed of PMDS A and B components in a ratio varying between 8:1 to 11: by weight. Ruby ball bearings can be employed as forms to make the molds for the solid immersion lenses. The fabricated molds are subsequently degassed and cured prior to use. Lens arrays as also fabricated using PMDS A and B components in a ratio varying between 8:1 and 11:1. The molded lenses are peeled from the RTV molds following a second degas and cure step.
The composite lens array is comprised of a Fresnel lens and two solid immersion lenses combined to make a double solid immersion lens with typical total magnification in the range of 10 to 100 times. It is possible using the methodology of the invention to fabricate lenses of xcx9cI cm2 area free of bubbles, macroscopic contaminants and other defects. Visible Spectroscopy absorption measurements on the composite lens indicate an extinction loss (absorption+scattering) of  less than 5% for photon energies between 1.3 and 3.3 eV, and infrared spectroscopy indicates relatively strong toluene-related absorption for photon energies less than 0.5 eV. These results clearly indicate suitability of PDMS lenses as optical elements of nonimaging microconcentrator lens arrays to enable flat plate microconcentrator photovoltaics.
In particular the invention is a method of fabricating a lens array for inclusion in a concentrator photovoltaic array comprising the steps of forming a plurality of Fresnel lenses and solid immersion lenses from optical grade silicone rubber. The plurality of Fresnel lenses and solid immersion lens are combined to form a compound optical system for focusing light at a corresponding plurality of locations. The compound optical system is disposed onto an array of photovoltaic cells such that light incident on the compound optical system is concentrated into each of the photovoltaic cells of the array.
The method of forming the plurality of solid immersion lenses comprises in turn for each solid immersion lens the steps of forming a cured bottom layer in a mold container, disposing a predetermined amount of molding material to form a second uncured layer on the bottom layer according to the desired height, h, of the solid immersion lens to be formed, embedding a spherical form into the second layer so that the spherical form rests on the bottom layer, curing the second layer with the spherical form in place, removing the spherical form from the cured second layer, disposing a predetermined amount of optical grade silicone rubber into the cured second layer according to the desired height of the solid immersion lens to be formed and according to the thickness of a base layer to be associated with the solid immersion lens, curing the optical grade silicone rubber in the second layer to form a cured solid immersion lens and base layer, and removing the cured solid immersion lens and base layer from the second layer.
The method of forming the plurality of Fresnel lenses comprises for each Fresnel lens the steps of forming a cured bottom layer in a mold container, pressing a Fresnel lens form into the bottom layer so that the lower surface of the Fresnel lens form is completely embedded into the bottom layer, completely covering the upper surface of the Fresnel lens form with molding material to form a second layer in which the Fresnel lens form is embedded, curing the second layer with the Fresnel lens form in place, removing the cured second layer, removing the Fresnel lens form from the bottom layer, disposing optical grade silicone rubber into a mold formed by the cured second layer and bottom layer, curing the optical grade silicone rubber in the mold to form a Fresnel lens, and removing the cured Fresnel lens from the mold. In the illustrated embodiment the plurality of Fresnel and solid immersion lenses are composed of GE Silicones RTV615 as the optical grade silicone rubber.
The method further comprises the step of cleaning the plurality of solid immersion and Fresnel lenses.
The predetermined amount of molding material to form the second uncured layer comprises disposing sufficient molding material in the mold container to form the second uncured layer with a depth substantially equal to the desired height, h, of the solid immersion lens, where h=R+R/nxe2x88x92a, where xe2x80x9cRxe2x80x9d is the radius of the spherical lens, xe2x80x9cnxe2x80x9d is the index of refraction of the cured optical grade silicone rubber, and xe2x80x9caxe2x80x9d is the thickness of the base layer associated with the solid immersion lens.
More specifically the step of forming the plurality of Fresnel and solid immersion lenses comprises mixing a silicone rubber compound and a silicone curing agent, and then defoaming the mixed silicone rubber compound and a silicone curing agent, namely mixing not more than 9 parts silicone rubber compound by weight to 1 part silicone curing agent by weight.
The step of forming the cured bottom layer in a mold container, and curing the second layer with the spherical form in place comprises degassing the bottom layer and then heating the degassed bottom layer, preferably for at least one hour in each case.
The method further comprises the step of forming a mold release layer on the cured second layer prior to disposing the predetermined amount of optical grade silicone rubber therein by plasma deposition.
Similarly, the steps of forming a cured bottom layer in a mold container, curing the second layer with the Fresnel lens form in place, and curing the optical grade silicone rubber in the mold to form a Fresnel lens comprise the steps of degassing and heating the bottom layer, second layer and optical grade silicone rubber in the mold respectively, preferably for at least one hour.
It must be understood also that the invention is a photovoltaic array comprising a plurality of Fresnel lenses and solid immersion lenses composed of molded optical grade silicone rubber forming a compound optical system for focusing light at a corresponding plurality of locations. The compound optical system is combined with an array of photovoltaic cells such that light incident on the compound optical system is concentrated into each of the photovoltaic cells of the array.
In one embodiment the Fresnel lens is combined with a single solid immersion lens to focus light for photovoltaic cell of the photovoltaic array. In another embodiment the Fresnel lens is combined with a double solid immersion lens to focus light for photovoltaic cell of the photovoltaic array. Again in the illustrated embodiment the plurality of Fresnel lenses and the plurality of solid immersion lenses are molded in an array from GE Silicones RTV615, or more generally the optical grade silicone rubber is comprised of a silicone rubber compound and a silicone curing agent, which are preferably mixed together in a ratio of not more than 9 parts silicone rubber compound by weight to 1 part silicone curing agent by weight.
While the apparatus and method has or will be described for the sake of grammatical fluidity with functional explanations, it is to be expressly understood that the claims, unless expressly formulated under 35 USC 112, are not to be construed as necessarily limited in any way by the construction of xe2x80x9cmeansxe2x80x9d or xe2x80x9cstepsxe2x80x9d limitations, but are to be accorded the full scope of the meaning and equivalents of the definition provided by the claims under the judicial doctrine of equivalents, and in the case where the claims are expressly formulated under 35 USC 112 are to be accorded full statutory equivalents under 35 USC 112. The invention can be better visualized by turning now to the following drawings wherein like elements are referenced by like numerals.